When bullying is alleged, it’s always important to take the allegations seriously. That means understanding that if bullying is occurring it will be ongoing and that there will be retaliation for it being reported.
The role of the coach is to coach both the alleged victim and the alleged bully in how to proceed and what their expectations are and what the consequences will be if retaliation occurs.
This means that both the alleged victim and bully must be encouraged to report everything that happens next (retaliation or whatever) so that the coach can deal with it.
What the coach then needs to do is investigate to see whether the incident in question happened or not. If it did, then the coach needs to assume that this is an ongoing problem and not just a one off incident and follow the rules in the student code of conduct, whatever that is.
If there is no proof the incident happened, they need to monitor it going forward. There is a good chance the bullying really is happening and by monitoring it and asking the victim to provide documentation on all future incidences, they should be able to get evidence of whatever the retaliation is (if it occurs) and take appropriate action as prescribed in the student code of conduct.
If this is a false accusation (they are rare, but they do occasionally happen), this ongoing reporting process will bring that to light and the coach can take appropriate action based on the student code of conduct.
What coaches should not do is assume that if you ask an alleged victim to document what all is happening, that when they do and they give you a lot of incidences (all little petty things), that they are not being truthful or that they are weak and petty. Those little things over time is what bullying looks like with the occasional big incident.
But again, the response should be in accordance with what the student code of conduct prescribes for transgressions.
Note: This question was asked of me on my free Udemy Course Why Bullies Bully & How to Stop Them. If you are suffering from bullying, please join this website to access all the free content and materials and get my book, The Bully Vaccine. If you are suffering from chronic bullying, sign up for the toolkit for help developing a strategy to get it to stop.